Objective: This study describes the epidemiology of hazardous materials (hazmat) incidents in Fresno County, California, and analyzes the emergency medical services (EMS) response to these incidents. Setting: The study area has a population of 635,000 people living in an area of 6,004 square miles. Design: All Hazmat Emergency Response Team (HERT) reports and related prehospital, emergency department, and inpatient records from 1 July 1988 through 30 June 1989 were reviewed retrospectively. Results: There were 107 hazmat incidents involving 156 materials consisting of pesticides (24.4%), miscellaneous chemicals (17.3%), corrosives (16.7%), petroleum products (13.5%), airborne toxins (10.2%), organic solvents (7.7%), unidentified chemicals (5.1%), infectious medical waste (1.9%), empty containers with radioactive warning symbols but without detectable radiation (1.3%), heavy metals (1.3%), and alkali metals (0.6%). In ten (9%) of the 107 incidents, 68 patients required on-scene evaluation and 26 patients were transported to emergency departments. Four of these patients required admission, three because of injuries due to vehicular accidents, and one because of a coincidental cerebrovascular accident. Five incidents produced multiple victims from exposures to airborne toxins, accounting for 63 (93%) of 68 patients. Spills of solid or liquid pesticides occurred in four incidents involving patients. Ambulance personnel and/or equipment became contaminated in three of these four incidents. Conclusions: 1) Ambulances should be dispatched selectively to hazmat incidents because only 9% of incidents involved patients. 2) training should emphasize personnel protection and proper patient decontamination to help prevent contamination of EMS personnel and equipment. 3) Preparation of EMS personnel should emphasize exposure to airborne toxins because these produced 93% of patients.